In manufacturing semiconductor devices, LSI packages are being assembled in increasingly higher densities through three-dimensional mounting or the like, and wafer thicknesses have been reduced to the order of 25 μm upon completion of processes. Furthermore, power devices such as IGBT (insulated gate bipolar transistor) and MOSFET (MOS field-effect transistor) are widely used as inverter circuits for industrial motors and automobile motors or the like, power supply apparatuses for large capacity servers and semiconductor switches for uninterruptible power supply apparatuses or the like.
These power semiconductor devices are manufactured by thinning semiconductor wafers to improve energization performance typified by on-characteristics or the like. In recent years, semiconductor devices are manufactured using an ultra-thin wafer process that reduces the thickness of a wafer material prepared using an FZ (floating zone) method down to the order of 50 μm to improve cost/characteristic aspects.
Generally, a wafer thinning process applies grinding by back grinding or polishing and wet etching or dry etching to remove processing distortion generated in mechanical polishing, and then forms a diffusion layer on the back side through ion implantation or heat treatment and forms electrodes using a sputtering method or the like. In such a situation, wafer cracking occurs more frequently during processing on the back side of the wafer.
Thus, there is a proposal of a processing method that processes only a central part of a wafer while causing a peripheral portion of the wafer to remain thick (e.g., see Patent Literature 1). Using a ribbed wafer in which such a rib structure is formed drastically reduces warpage of the wafer and facilitates a wafer transfer by a process apparatus. Moreover, this drastically increases the strength of the wafer and reduces cracking or chipping of the wafer while handling the wafer. Moreover, there is a proposal of a method whereby grinding water made of carbon dioxide and pure water during a grinding process is supplied to such a ribbed wafer to thereby prevent charging of the grinding water and prevent adhesion of foreign substances to the grinding surface (e.g., see Patent Literature 2). In transferring the ribbed wafer, there is a proposal of a method of preventing adhesion of foreign substances by tilting the inner circumferential wall of the rib into a reverse tapered shape and supporting the side wall from inside (e.g., see Patent Literature 3).